Tropical Storm IRWIN

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TROPICAL STORM IRWIN DISCUSSION NUMBER 31
NWS NATIONAL HURRICANE CENTER MIAMI FL EP112011
200 PM PDT THU OCT 13 2011
DEEP CONVECTION HAS WANED NEAR THE CENTER OF IRWIN DURING THE PAST
FEW HOURS...HOWEVER 1800 UTC DVORAK ESTIMATES FROM TAFB AND SAB
SUPPORT HOLDING THE INITIAL INTENSITY AT 35 KT FOR THIS ADVISORY.
THE OVERALL INTENSITY FORECAST REASONING REMAINS UNCHANGED...WITH
LITTLE CHANGE EXPECTED FOR THE NEXT DAY OR SO AS IRWIN CONTINUES
MOVING THROUGH A RELATIVELY DRY MID-LEVEL ENVIRONMENT DESPITE BEING
SITUATED OVER VERY WARM SSTS. WHILE MOST OF THE GUIDANCE CONTINUES
TO SHOW STRENGTHENING AT 48 HOURS AND BEYOND...THE OFFICIAL
FORECAST WILL SHOW ONLY A SMALL INCREASE IN INTENSITY AT 2 OR 3
DAYS. A LITTLE WEAKENING IS FORECAST LATE IN THE PERIOD AS THE
VERTICAL SHEAR IS EXPECTED TO INCREASE. THE NHC FORECAST IS BELOW
THE INTENSITY CONSENSUS THROUGH THE PERIOD AND IS CLOSE TO THE HWRF
MODEL.
IRWIN HAS SLOWED AND TURNED TOWARD THE EAST OVER THE PAST FEW
HOURS...AND THE INITIAL MOTION ESTIMATE IS 090/05. A MID-LEVEL RIDGE
CURRENTLY CENTERED NEAR 30N 120W WILL BUILD SOUTHEASTWARD OVER THE
NEXT COUPLE OF DAYS...WHICH SHOULD FORCE A IRWIN TO TURN SHARPLY
TOWARD THE SOUTH AND THEN TOWARD THE SOUTH-SOUTHWEST IN THE NEXT 48
HOURS. FOR THIS CYCLE THE GUIDANCE HAS TRENDED AGAIN TOWARD A SHAPER
SOUTHWARD TURN IN THE SHORT TERM WITH A SLIGHTLY FASTER FORWARD
MOTION...AND SO HAS THE NHC FORECAST. BY 72 HOURS THE RIDGE BEGINS
TO SHIFT NORTHWARD AND LEAVES IRWIN IN A REGION OF WEAK STEERING
CURRENTS. NOT SURPRISINGLY THE SPREAD IN THE TRACK GUIDANCE
INCREASES MARKEDLY AT DAYS 4 AND 5...WITH THE GFS AND UKMET SHOWING
A TURN BACK TOWARD THE EAST WHILE THE ECMWF SHOWS A MUCH WEAKER
IRWIN MOVING MORE QUICKLY WESTWARD. THE NHC FORECAST SHOWS A SLOW
WESTWARD MOTION AT THE END OF THE PERIOD DUE TO THE LARGE
UNCERTAINTY...AND HAS BEEN ADJUSTED A LITTLE TO THE SOUTH TOWARD
THE NEW MULTI-MODEL CONSENSUS.
GIVEN THE SMALL AREA OF TROPICAL STORM FORCE WINDS AND THE EXPECTED
SHARP TURN TOWARD THE SOUTH OVER THE NEXT COUPLE OF DAYS...TROPICAL
STORM WATCHES OR WARNINGS ARE NOT REQUIRED FOR THE COAST OF MEXICO
AT THIS TIME.
FORECAST POSITIONS AND MAX WINDS
INIT 13/2100Z 18.6N 106.5W 35 KT 40 MPH
12H 14/0600Z 18.4N 106.1W 35 KT 40 MPH
24H 14/1800Z 17.5N 105.7W 35 KT 40 MPH
36H 15/0600Z 16.5N 105.7W 35 KT 40 MPH
48H 15/1800Z 15.5N 106.1W 40 KT 45 MPH
72H 16/1800Z 14.5N 107.5W 40 KT 45 MPH
96H 17/1800Z 14.5N 108.5W 35 KT 40 MPH
120H 18/1800Z 14.5N 109.5W 35 KT 40 MPH
$$
FORECASTER BRENNAN
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